Gas atomization of a fluid such as a paint composition to break up the fluid into particles for subsequent application to a workpiece to be coated is a technique well recognized in the art. Spray apparatus generally employed is a spray gun to which is supplied a fluid stream and a gas stream. The gas is most usually air, but can, of course, be chosen from other gases as required. The fluid stream issues from the spray gun via a fluid nozzle while the gas stream issues via a gas nozzle, with the gas stream intersecting or otherwise disturbing the fluid stream to provide atomized sprayed fluid particles.
To improve coating characteristics of the fluid particles issuing from the spray gun, various techniques have been developed to electrostatically impart an electrical charge to these particles prior to their arrival on the workpiece to be coated. One such technique is induction charging. Briefly, and in relation to the instant invention, a method of inducing an electrical charge on sprayed fluid particles involves the placement of an induction charging electrode means adjacent the fluid and gas nozzles. This electrode means induces an electrical charge on the atomized fluid particles, which charge is opposite to the electrode's charge, as the particles pass within a charging zone created between the electrode means and the particle stream. The electrode means itself can be an integral fixture of the spray gun, or it can be removably connected to the spray gun. An example of the latter electrode means which can be fitted to a conventional spray gun is described in U.S. Pat. No. 4,009,829, to James E. Sickles, incorporated herein by reference.
A second technique for imparting an electrostatic charge to fluid particles is corona charging. In this technique a needle-like electrode is disposed in the stream of fluid prior to atomization of the fluid into particles. The electrode discharges an electrical charge which is held by the fluid, with the subsequently formed fluid particles thus having a charge of the same polarity as that of the corona electrode. Voltage requirements in a corona charging system are, however, relatively high, generally 50 to 60 KV, and therefore create possible safety and energy-consumption disadvantages.
Copending application Ser. No. 076,014, filed on even date herewith and entitled "Electrostatic Spray Gun Having Increased Surface Area From Which Fluid Particles Can Be Formed," discloses a spray gun having disposed within the fluid stream means for increasing surface area from which fluid particles can be formed, said means being electrically grounded at least during fluid issue from the nozzle. Said spray gun has electrostatic charging means comprising an induction charging electrode means disposed adjacent the gas and fluid nozzles to create a charging zone wherein an electrical charge is induced on formed fluid particles. As related in said copending application, the surface area increasing means acts to provide a surface area for fluid particles issuing from the fluid nozzle orifice to be better exposed to the charging zone, and, because said means is grounded, to create a favorable potential gradient between the fluid particles and the electrode. Charging of the fluid particles therein described is solely accomplished by induction charging. It is known, however, that some fluids have a relatively medium-to-low electrical conductivity, generally defined as below about 0.06 .mu.mho/cm. It is also known that a spray stream contains fluid particles whose sizes cover a range from large to small. Further, it has been found that larger particles having such medium-to-low electrical conductivity are not as well charged with induction charging as are those particles whose electrical conductivity exceeds about 0.06 .mu.mho/cm. Smaller particles having medium-to-low electrical conductivity are, however, adequately charged to high charge-to-mass ratios. Conversely, it has been found that said larger particles do obtain adequate charging from a corona discharge means, while the smaller particles do not find optimum benefits with corona charging.